1. Field of the Invention
The invention relates to a voltage to current converting circuit, and more particularly to a voltage to current converting circuit that is capable of operating at a low voltage.
2. Description of the Related Art
In analog circuits, a transconductance circuit is a voltage to current converting circuit, which converts an input voltage into an output current for subsequently other circuits.
FIGS. 1A and 1B show a single-end mode and a differential mode for a conventional transconductance circuit, respectively. In FIG. 1A, a transistor M1 is coupled to a ground GND via a resistor R. An input voltage Vi is used to control a gate of the transistor M1, to determine a current value of an output current io flowing through the transistor M1. In FIG. 1B, a transistor M1 is coupled to the ground GND via a first current source, and a transistor M2 is coupled to the ground GND via a second current source, wherein the first and second current sources have the same current values I0. In addition, a resistor R is coupled between the drains of two transistors M1 and M2. The input voltages Vi+ and Vi− are a pair of differential signals, that are used to control the gates of the transistors M1 and M2, to determine a current value of the output current io+ flowing through the transistor M1 and a current value of the output current io− flowing through the transistor M2. In the conventional transconductance circuit, the resistor R is much larger than the transconductance gm of each transistor, i.e.
      R    ⪢          1      gm        ,so as to obtain better linearity. Furthermore, the conventional transconductance circuit needs to operate at an operating range having a good linearity as the input voltages Vi+ and Vi− are applied to the gates of the transistors M1 and M2 directly. However, the operating range is decreased when a supply voltage is decreased.
FIGS. 2A and 2B show a single-end mode and a differential mode for another conventional transconductance circuit, respectively. In FIG. 2A, a transistor M1 is coupled to the ground GDN via a resistor R, wherein a gate of the transistor M1 is coupled to an output terminal of an amplifier AMP1. By using a characteristic of virtual short between two input terminals of the amplifier AMP1, the voltages at two terminals of the resistor R are an input voltage Vi and the ground GND, thereby an output current io is obtained by applying the input voltage Vi into the resistor R, i.e.
      i    0    =                    V        i            R        .  In FIG. 2B, a transistor M1 is coupled to the ground GND via a first current source, and a transistor M2 is coupled to the ground GND via a second current source, wherein the first and second current sources have the same current values I0. A gate of the transistor M1 is coupled to an output terminal of an amplifier AMP1, and a gate of the transistor M2 is coupled to an output terminal of an amplifier AMP2. Furthermore, a resistor R is coupled between the first terminals of the amplifiers AMP1 and AMP2. The input voltages Vi+ and Vi− are a pair of differential signals, wherein the input voltages Vi+ and Vi− are applied to the second terminals of the amplifiers AMP1 and AMP2, respectively. Similarly, by using a characteristic of virtual short between two input terminals of each of the amplifiers AMP1 and AMP2, the output currents io+ and io− are obtained by applying the input voltages Vi+ and Vi− into the resistor R. Although the conventional transconductance circuits of FIGS. 2A and 2B use the amplifiers to overcome the problems of the conventional transconductance circuits of FIGS. 1A and 1B, the amplifiers AMP1 and AMP2 must maintain in the virtual short status thereof, so as to maintain better linearity. However, the operating range of the virtual short status is decreased for an amplifier when a supply voltage of the amplifier is decreased, thus it is hard to maintain linearity.
Following the advancement of process technology, integrated circuits (IC) can operate at a lower supply voltage, such as below 1.5V, so as to decrease power consumption for the IC. However, when the operating/supply voltage is decreased, the linearity of each conventional transconductance circuit of FIGS. 1A, 1B, 2A and 2B is decreased, and can not meet operating requests.
Therefore, a voltage to current converting circuit having better linearity is desired, that is capable of operating at a low voltage.